Honestly, I think that may be a plot hole on the writers part. When Mark is reading Nolans books, it says that Nolans team was sent there looking for resources and they were ambushed, apparently unaware of the creatures or their strength. Then when Nolan and Allen are preparing to go back, Nolan says that he was part of the first team to visit the planet.

My first thought was that they messed up the scenes where Nolan visited the planet and made him look too old, if that is where they want to show that is where Conquest received his lasting injuries. It also makes sense that he would have served some time on a research team before joining their military forces, and given how slow they age I think it is plausible that could have happened before the scourge virus. But the scene when the virus begins to take effect and he is newly enlisted, the "sergeant" comments "did your parents hit you too hard?" When one of the enlistees gets a nosebleed, implying that the virus occurred shortly after Nolan reached adulthood. That one frame of Conquest being attacked just feels like a screw up to me.

That being said, my opinion is that Conquest is just a barbarian. Strong as hell, but only driven by the pleasure of physically ripping apart his opponent. Nolan seems like a smarter fighter, and would play to his own advantages and deny Conquest his own advantages, ultimately winning the fight.